Botulinum neurotoxins are a family of bacterial toxins, including seven major serotypes (BoNT/A-G)1. These toxins act by blocking neurotransmitter release from neurons, thus paralyzing animals and humans. In recent years, BoNTs have been widely used to treat a growing list of medical conditions: local injections of minute amount of toxins can attenuate neuronal activity in targeted regions, which can be beneficial in many medical conditions as well as for cosmetic purposes2-4.
BoNT/A and BoNT/B are the only two BoNTs that are currently FDA-approved for use in humans2-4. These are toxins purified from bacteria without any sequence modifications (defined as wild type, WT). As the application of BoNTs grows, limitations and adverse effects have been reported. The major limitation is the generation of neutralizing antibodies in patients, which renders future treatment ineffective5. Termination of BoNT usage often leaves patients with no other effective ways to treat/relieve their disorders. The possibility of antibody responses is directly related to both toxin doses and the frequency of injection5. Therefore, this limitation mainly occurs in treating muscle spasms, which involves relatively high doses of toxins. Consistently, antibody responses have not been observed in cosmetic applications, which use extremely low toxin doses5.
The major adverse effects are also often associated with treating muscle spasms, but not cosmetic applications. This is because the adverse effects are largely due to diffusion of toxins to other regions of the body and the possibility of toxin diffusion is directly related to injected doses. The adverse effects ranges from transient non-serious events such as ptosis and diplopia to life-threatening events even death6,7. In a petition letter filed in 2008 by Dr. Sidney Wolfe to FDA, a total of 180 serious adverse events, including 16 deaths have been documented. As a result, FDA now requires the “Black box warning” on all BoNT products, highlighting the risk of the spread of toxins, following similar warnings issued by the European Union.
Because both the generation of neutralizing antibodies and toxin diffusion are directly related to injected doses, lowering toxin doses (while maintaining the same levels of toxin activity) is highly desired, which means the efficacy of individual toxin molecules has to be enhanced. Such modified BoNTs with improved specificity for neurons will also reduce any potential off-target effects due to non-specific entry into other cell types.
BoNTs target and enter neurons by binding to their specific receptors through their receptor binding domains, which are well-defined in the literature (BoNT-HC, FIG. 1A, B)1. Receptor binding dictates the efficacy and specificity of BoNTs to recognize neurons. Improving the receptor binding ability of BoNTs will enhance their efficacy and specificity to target neurons. The receptors for most BoNTs have been identified (FIG. 1C). BoNT/B, D-C, and G share two homologous synaptic vesicle proteins synaptotagmin I and II (Syt I/II) as their receptors8-13, while BoNT/A, E, D, and F use another synaptic vesicle protein SV29,14-18. In addition to protein receptors, all BoNTs require lipid co-receptor gangliosides (FIG. 1D), which are abundant on neuronal surfaces19. Among the two Syt isoforms in rodents and likely in most mammals, Syt II has ˜10-fold higher binding affinity for BoNT/B than Syt I and is also the dominant isoform expressed in motor nerve terminals, which are the targeted neurons for BoNTs (FIG. 2A)20,21. Therefore, in rodents (on which most research has been conducted), Syt II is considered the major toxin receptor, while Syt I is a minor toxin receptor at motor nerve terminals.
One may argue that BoNTs already have high specificity to neurons, is it possible to further improve their binding to neurons? The answer is a “Yes” for humans, because it was recently discovered that the human Syt II has greatly diminished binding and function as the receptor for BoNT/B due to a unique amino acid change from rodent (rat/mouse) Syt II within the toxin binding site13,22. This is a change from phenylalanine (F) to leucine (L) at position 54 (mouse Syt II sequence) (FIG. 2B). Sequence alignments have revealed that phenylalanine at this position is highly conserved in both Syt I and Syt II across vertebrates, including platypus, fish, rodents, and monkeys23. Only human and chimpanzee Syt II contains leucine at this position. As a result of this residue change, human and chimpanzee Syt II has greatly diminished binding to BoNT/B, D-C, and G (FIG. 2C) and is significantly less efficient in mediating the entry of BoNT/B (FIG. 2D), as compared to mouse Syt II. Since human and chimpanzee Syt I still contains phenylalanine at the same position and can bind BoNT/B, D-C, and G (FIG. 2E), the high affinity receptor for BoNT/B, D-C, and G in humans is restricted to the minor receptor Syt I. These findings provide an explanation for the clinical observations that a much higher dose of BoNT/B than BoNT/A (which binds a different receptor) is needed to achieve the same levels of therapeutic effects in patients24,25. Previously these observations were attributed to other reasons, such as the percentage of active neurontoxin in the preparations used. The recent observations of such binding differences of BoNT/B and human Syt II versus Syt II of other species suggests that different residues of BoNT/B may be involved in binding to human Syt II. As such, sequence modification to BoNT/B that is expected to affect binding to rodent SytII may have unpredictable affects on BoNT/B binding to human Syt II.